In a variety of manufacturing and production settings, there is a need to control alignment between various layers or within particular layers of a given sample. For example, in the context of semiconductor processing, semiconductor-based devices may be produced by fabricating a series of layers on a substrate, some or all of the layers including various structures. The relative position of these structures both within a single layer and with respect to structures in other layers is critical to the performance of the devices. The misalignment between various structures is known as overlay error.
The measurement of overlay error between successive patterned layers on a wafer is one of the most critical process control techniques used in the manufacturing of integrated circuits and devices. Overlay accuracy generally pertains to the determination of how accurately a first patterned layer aligns with respect to a second patterned layer disposed above or below it and to the determination of how accurately a first pattern aligns with respect to a second pattern disposed on the same layer. Presently, overlay measurements are performed via test patterns that are printed together with layers of the wafer. The images of these test patterns are captured via an imaging tool and an analysis algorithm is used to calculate the relative displacement of the patterns from the captured images. Such overlay metrology targets (or ‘marks’) generally comprise features formed in two or more layers, the features configured to enable measurement of spatial displacement between features of the layers (i.e., the overlay or displacement between layers).
In a standard metrology target design, each target layer is assigned at least two pattern elements (e.g., square, rectangle and the like) that have a single center of symmetry. The centers of symmetry of a target structure of a first layer and a target structure of a second layer are designed such that they are at the same location when overlay is zero (i.e., target structures of each layer are aligned). In settings where non-zero overlay exists, the center of symmetry of one layer is shifted with respect to the center of symmetry of the second layer. Typically, in order to determine the symmetry point of each layer, a region of interest (ROI) is generated that surrounds each pattern element of each target structure of each layer. Therefore, the entire target includes the multiple target structures consisting of the various ROIs needed to characterize the constituent pattern elements of the target structures of the target, whereby the overall area of the target is determined by the size of the various structures of the target.
Further, in some instances, process design rules for the sample (e.g., wafer) require the use of segmented pattern elements of the target structures of the target. In the case of a target including segmented pattern elements, overlay is reliable in only one direction, the direction perpendicular to the segmentation lines of the pattern elements (e.g., ‘thin’ rectangle parallel aligned lines). Therefore, in order to adequately measurement overlay in both the X- and Y-directions target area must be increased by a factor of two to account for the needed additional target structures.
FIG. 1A illustrates a known overlay target 100 having 90 degree rotational symmetry, respectively, about a center of symmetry 110. Each of the target structures of FIG. 1A include pattern elements (e.g., 102a through 104b), which are individually invariant to 90 degree rotation. Due to the 90 degree invariance of the individual pattern elements, the pattern elements of targets 100 of FIGS. 1A are suitable for both X-overlay and Y-overlay measurements.
FIG. 1B illustrates target 150 which displays invariance to a 90 degree rotation. In contrast to FIG. 1A, the individual pattern elements (e.g., 202a through 204d) display only 180 degree rotational symmetry. As such, at least two separate orthogonally oriented pattern elements must be used in order to measure overlay in both the X- and Y-direction. For instance, the pattern elements 202a, 204a, 202d, and 204d may be used to measure overlay in a first direction, while elements 202b, 204b, 204c, and 202c may be used to measure overlay in a second direction orthogonal to the first direction. Due to the increased target structures required to perform multi-directional overlay measurements, additional space is needed on a given sample (e.g., wafer) to accommodate the additional target structures. It is therefore advantageous to provide a metrology target and a system and method for implementing such a metrology target that cures the defects of the prior.